The U.S. military has been hit with a series of allegations that soldiers and Marines in Iraq committed atrocities against civilians during the last several months. The allegations have resulted in investigations, criticism and much speculation about why this series of incidents appears to have happened in a period of just a few months.

The allegations are horrific, that U.S. Marines killed 15 civilians in the town of Haditha last November, including seven women and three children, apparently in a rage over the death of one of their comrades; that another group of Marines killed an Iraqi man in Hamdaniya in April, when they were looking for another man; that five U.S. Army soldiers entered a home in Mahmoudiya in March, murdered a family of four, and raped the older daughter, who may have been as young as 15.

Pentagon spokesman Bryan Whitman agrees that, if the reports are true, the incidents were terrible, but he says the official criminal investigations have not yet been completed.

"Every allegation of wrongdoing by U.S. military forces is taken very seriously," he said. "It's investigated thoroughly, and when we find people that have done something that's inconsistent with the values and laws of this country, we take appropriate action and hold people accountable."

Several Marines and soldiers have been taken into custody while the investigations continue. And the Commandant of the Marine Corps took the allegations so seriously that he went on a hastily arranged visit to Iraq to speak to his troops about upholding basic human values, even as they fight an enemy that likes to hide among civilians.

Officials will not speculate about why these alleged incidents have apparently happened. Outside analysts suggest it could be related to the stress of combat, the length of the war in Iraq, training deficiencies and many other factors. But spokesman Bryan Whitman is reluctant to accept any such connections.

"We should be careful about making connections to things like you indicated," he added. "There's no evidence to suggest that at this point. But we don't have to guess at that. We'll know soon enough, when the investigators are completed with their work. And if there are systemic things to address, you can be sure that the military will."

Retired Marine Corps Major General Thomas Wilkerson offered his own ideas.

"There are times when, in the heat of battle, people lose track of their situation," he explained. "And if they did, that could be one reason. The other thing is, they can just be bad people."

The retired general says the military, like society as a whole, simply has some people who turn out to be criminals, and that tendency could be brought out by what he calls the 'heat of battle.' He also cautions against jumping to conclusions about how the victims died.

"If someone saw the shooting and can verify that, in fact, it was something completely outside of the realms of the rules of engagement, so be it, but many times it's a very muddy picture," he added.

General Wilkerson says not every civilian death in war is the result of misconduct or criminal activity by the troops.

"There are some who say, 'look, they died and they shouldn't have died. Someone must be responsible, and in order for them to have been killed someone must have violated the rules.' Not true," said General Wilkerson.

At Yale University Medical School, Psychiatrist Deane Aikins is working on a research project with the U.S. Army to determine the impact of stress on soldiers. The study is still in its early stages, but he says he has already determined, through brain imaging, that some soldiers physically deal with the stress of battle better than others. He says troops have to deal with what is called Post-Traumatic Stress Disorder, or PTSD, even while they are still in the stressful environment of combat. But he says such stress is usually internalized.

"The PTSD doesn't make you go and become homicidal. It's very internalized anguish," said Dr. Aikins.

Dr. Aikins says he is looking for physical traits revealed in the brain images that might help predict whether a person will be a good soldier. But he says he and other researchers have not found any such traits yet.

"Can we predict who will perform poorly? There is 60 years of research that say we can't, with combat service. So, this idea of can we predict who would commit a war atrocity, we don't know how to do that. And part of it is because, look, it's so rare," added Dr. Aikins.

So far, Professor Aikins has worked with soldiers who served in Iraq and have post-traumatic stress, and some who do not, but he has not had the chance to examine anyone charged with committing an atrocity.

His work will continue, as will the military investigations of these alleged incidents. U.S. officials emphasize that hundreds of thousands of U.S. troops have served honorably in Iraq, and more than 2,500 have died trying to secure the country's freedom. But they also say, if any of the alleged atrocities are proved, the men responsible will be prosecuted, and if there are any indications of steps commanders can take to prevent future incidents, those steps will be taken.